Silica (chemically known as silicon dioxide) is one highly abundant chemical compound found in earth crust. There exist large variety of silica based materials (SBMs) in nature, either in form of sand or glass or zeolites or quartz or even in microorganisms like diatoms. Like nature, also researchers have synthesized large variety of SBMs in laboratories. These SBMs are micro or meso or macroporous in nature and have myriad of potential applications in several fields such as sensing, catalysis, bio imaging, biomolecular (or drug) delivery, photovoltaics, optics and optoelectronics etc. The first question which may arise is about the reason(s) behind widespread applications of SBMs.
- Sol-gel synthesis of silica: To address above mentioned key question about SBMs, one should discuss the sol-gel polymerization method to synthesize silica. In 1968, W. Stöber and A. Fink have reported a room temperature method to synthesize silica (popularly known as Stöber method) (Figure 1). This procedure is also named as sol-gel method, where a transition occurs from sol to gel in two steps: (1) hydrolysis of liquid alkoxide precursor (called sol) in presence of catalytic amount of acid or base to tetrahydroxysilane monomer and (2) polycondensation of tetrahydroxy monomers to silica gel. Sol-gel is the most common, simple and cheap chemical methodology adopted to synthesize SBMs After polymerization Si-O-Si network terminates exposing silanol (Si-OH) groups to the local environment; these Si-OH plays the main magical role behind applications of silica nanostructures.
Figure 1. A schematic representation of Stöber method. =Si(OR)2: alkoxide precursor; =Si(OH)2: silicic acid monomer, ROH: alcohol and H2O: water.
Figure 2. Trapping of guest molecules via non-covalent host-guest interaction
Nature of silica surface: Silica nanoparticle is basically a 3D network of Si-O-Si moieties, where each Si atom is tetrahedrally connected to four neighbouring oxygen atoms. However, large surface area of nanoparticle consists Si-OH groups of different kinds viz. isolated (or free) Si-OH, vicinal HO-SiOSi-OH, geminal HO-Si-OH, and H-bonded Si-OH·····OH-Si. Due to polar nature of –O-H bonds, the surface of silica is polar-protic in nature. Surface Si-OH groups are already known to be potent for hydrogen bonding or dipole-dipole interaction. Porous nanostructures of silica or SBM (for example zeolites) nanoparticles are well capable to host guest molecules via hydrogen-bonding or dipolar interaction. Ability of silica pores to adsorb water molecules has already opened up a large scale applications as dehydrating agents. Due to trapping ability, nanostructures of SBM can act as heterogeneous catalyst; hence find ample catalytic applications in chemical and petroleum industry. Moreover, fluorescent guests like luminescent nanoparticles or organic fluorophores seat inside pores; these adsorbed molecules does not leach out easily even after rigorous washing. Fluorophore loaded hybrid SBM nanomaterials can be applied for bio-imaging or optics. However, there remains scopes of improvement via covalent surface modification.
fluorophores seat inside pores; these adsorbed molecules does not leach out easily even after rigorous washing. Fluorophore loaded hybrid SBM nanomaterials can be applied for bio-imaging or optics. However, there remains scopes of improvement via covalent surface modification.
- Surface modification via covalent functionalization: Surface modification enables one to tag molecules of interest (IM) to silica surface. The most important strategy to modify silica surface is covalent functionalization, which is achieved using a linker. The most common linker used is 3-aminopropyltrimethoxysilane (APTMS) or 3-aminopropyltriethoxysilane (APTES). One can modify the surface of silica (SMS) via two ways. (1) The first methodology is via cocondensation between linker and silica surface to form linker tagged silica nanoparticle (LTS), followed by covalent tagging of IM with surface- amino groups or (2) The second methodology is via covalent bonding between linker and IM to form a conjugate followed by the cocondensation of the conjugate and silica surface, (given below).
The reaction between amino group and IM occurs via one-step click reaction of high yield. Surface modified silica nanoparticles can be obtained in pure state simply by washing. Several washing ensures removal of unwanted molecules or covalently attached IMs.
- Nature of IM: Surface modification methodology is adopted to serve several purposes.
(i) To modify silica surface with suitable solvent philicity: One can use a linker with a hydrophobic chain like n-propyltrimethoxysilane (PTMS) instead of APTMS and can modify the hydrophilic surface to hydrophobic one (using just first step of methodology 1). Silica nanoparticles with hydrophobic (or nonpolar) surface can act as water (or polar solvent) repellant. Thus, it is possible to tune surfaces of silica nanoparticles to lyophilic or lyophobic, which finds applications in printing or paint industry, devices and for delivery of drugs or biomolecules (ii) As biomarker and sensing: As discussed above, fluorophores or quantum dots, or nanoparticles can be attached to the surface of silica, hence employable for bioimaging, optoelectronics, photovoltaics etc. The fluorophore modified silica nanoparticles are applied for pH or metal ion. Tagging of suitable receptors like aptamers, antibodies enable successful applications of silica nanoparticles for the targeted drug delivery or biosensing purposes. (iii) Silica spacing around inorganic or organic structure: A thin layer of silica provides protection to several species like lead based perovskite or organic nanoparticles etc. Silica shell around Si or any semiconducting nanostructures are crucial due to their insulating nature and ease of surface modification.
- Scope(s) of research based on silica: Apart from the role ofSi-OH groups, there are several other factors behind the demands of SBMs. Biocompatibility is one such reason allowing their bio related applications. Often, silica is used as nanoshell to avoid adverse effect of toxic core materials during bioinvestigations. Nanostructures of SBMs are low density, highly porous and colloidal in nature. It is easy to disperse them in suitable solvent or deposit as layer for optical characterization or for device fabrication. They are chemically and thermally stable. Thus the risk of chemical and structural destruction of SBMs are low, while using them as catalyst or in devices. Quartz or defect free silica are optically transparent with high band-gap; they are necessary for different components in optical devices. Photophysical investigations reveal the presence of defects in quartz, which emit only at VUV excitation. Thus, quartz based optical investigations can only be interfered at VUV or UV excitation, but not in lower energy UV-VIS-IR regions. Silica based hybrid materials with the emission from conjugated component are investigated photophysically. One important type of hybrid material is Si/SiO2 nanostructures, which are applied in electronic devices like Si based MOS FET.
Covid-19 pandemic has exposed human races not only towards a health crisis, but also to a large economic crisis. Looking at that scenario, cost effectiveness of any materials or methodology will be a concern for industrial applications. Due to large abundance in nature and ease of synthesis, SBMs are cheap. One can easily recover SBMs catalysts while wastes can be easily disposed. Thus, we can conclude that pure or hybrid SBMs will be “in high demand” materials to the scientific community for ever.
References:
(1) Banerjee et al. Silica based materials for bioanalytical chemistry and optoelectronics: Chemistry of silica and zeolite based materials, Douhal, A.; Masakazu, A., Elsevier: Amsterdam, Netherland, 2019, 213-228.
(2) Banerjee, S. et al. J. Phys. Chem.C 2011, 115, 1576.
(3) Banerjee et al. Small 2016, 12, 5524.
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